Data centers or large clusters of servers have become increasingly employed in universities, enterprises and consumer settings to run a variety of applications such as web services, instant messaging, gaming, data analysis, scientific computing and many others. Data centers typically comprise many thousands of servers arranged hierarchically, typically with racks containing 10-40 servers each, linked by a Data Center Network (DCN). FIG. 1 is a schematic diagram of a traditional data center network. The data center (1) comprises a link to external networks (2), servers (6) and a switching hierarchy (7) comprising core routers (3), access routers (4), switches (5) cross point switches (8).
A major problem in such networks lies in large data flows, known as “elephant flows”, which typically originate from server back-up, cloning, load balancing, backup, Virtual Machine migrations, or communication among memory banks and CPU in hyper scales data centers. Elephant flows are comparatively rare, but when they are present, they can dominate a data center network at the expense of smaller so-called “mice flows”. This can have a highly detrimental effect on the quality of service of mice flows, which are typically delay sensitive. FIG. 2 is a graphical representation of “elephant” and “mice” flows. The graph shows network load (9) against time (10), with mice flows (11) below a threshold (12) and elephant flows (13) above the threshold. Although relatively rare, elephant flows cause major problems as they dominate the network and are highly detrimental to delay and delay variation in smaller flows.
One solution to this problem is the use of “packet offload”, wherein a dedicated optical network is provided for elephant flows. Offload enables transit traffic to optically bypass intermediate packet processing entirely, resulting in reduced required capacity and optimized power consumption. Moreover, capacity of optical channels allows accommodating bandwidth hungry data transfers.
FIG. 3 is a schematic diagram of an optical offload network for a data center network according to the prior art. In addition to the usual electrical switching arrangements (7), there is an optical network (14). Each rack of servers has a top of rack (ToR) switch (15), each of which is connected to the optical network (14). The optical network comprises an optical cross-connect (16) in the form of a Micro-electrical mechanical switch (MEMS).
Although effective in providing an optical offload, such a network is expensive to implement. There is also a lack of flexibility in providing point to multipoint connection to allow one node to communicate simultaneously with more than one other node.